> -----Original Message-----
> From: Keir Fraser [mailto:Keir.Fraser@xxxxxxxxxxxx]
> Sent: 26 January 2007 10:14
> To: Petersson, Mats
> Cc: Xen Development Mailing List
> Subject: Re: [Xen-devel] [PATCH] 32on64 call gate support
>
> On 26/1/07 10:07 am, "Petersson, Mats" <Mats.Petersson@xxxxxxx> wrote:
>
> >> Do we know that anyone uses 64-bit call gates? I would
> have thought it
> >> unlikely.
> >
> > I agree, it's VERY unlikely that anyone uses call-gates in
> 64-bit mode.
> > There are other, better, ways to do system calls, including the
> > dedicated SYSCALL instruction. Since 64-bit code is "new",
> it's unlikely
> > to use old ways of doing things "just because there was nothing else
> > around at the time".
>
> Of course, it begs the question why call gates weren't
> stripped from long
> mode along with other ancient architectural baggage.
Yeah, well... Actually, there may be a good reason for call-gates: If
you use call-gates correctly, you could avoid having one arugment that
is "which system call", but rather set up three dozen call-gates for
your three-dozen system calls - so that may be what they were preserving
it for, along with your suggestion below about old-OS support. There are
a few more possible call-gates than other methods of system-call, even
if they are not quite "unlimited". [Bear in mind that I'm NOT a
chip-architect, so I don't know why they kept some things and removed
others, aside from the "prefix" clean-up]
>
> Perhaps the idea was to allow compat-mode applications
> relying on call gates
> to be supported? So the OS would be responsible for
> allocating 64-bit call
> gates instead of 32-bit call gates?
>
> Anyhow, this is a bridge we should probably only cross if we
> come to it.
Definitely my opinion too.
--
Mats
>
> -- Keir
>
>
>
>
>
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